Sintered electroconductive oxides having electrical conductivity and changing in resistance (specific resistance) with changes in temperature, thermistor elements using the sintered electroconductive oxides and temperature sensors using the thermistor elements are conventionally known.
Patent Document 1 discloses a sintered material for a thermistor element, which consists of Sr, Y, Mn, Al, Fe and O and has a crystal phase of a perovskite type oxide, a crystal phase of a garnet type oxide and a crystal phase of at least either a Sr—Al oxide or a Sr—Fe oxide, to enable temperature measurements over the range of 300° C. to 1000° C.
Patent Document 2 discloses a sintered electroconductive oxide material for a thermistor element, which has a composition of M1aM2bM3cM4dO3 where a, b, c and d satisfy given conditional expressions, to show a suitable specific resistance over the range of room temperature to 1000° C.
Patent Document 3 discloses a thermistor element formed with a mixed sintered material (MM′)O3.AOx of a composite perovskite type oxide (MM′)O3 and a metal oxide AOx.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-221519
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-183075
Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-143907
One use of a thermistor element is as a temperature sensor designed to detect the temperature of an exhaust gas from an automotive engine such as internal combustion engine. For protection of DPF and NOx reduction catalyst, there has recently been a demand for the thermistor element to enable temperature measurements in a high temperature range of around 900° C. There has also been a demand for the thermistor element to enable temperature measurements in a low temperature range e.g. under an engine starting or key-on state for detection of temperature sensor failure (wire break) in OBD systems (On-Board Diagnostic systems). It is especially demanded that the thermistor element be capable of carrying out temperature measurements even at a temperature of −40° C. since there may be a case where the engine starting temperature becomes below the freezing point in cold climates.
However, each of the sintered materials of Patent Documents 1 and 2 have a temperature gradient value (B-value) of about 4000 K or higher so as to show a suitable resistance change over the range of room temperature or not lower than 300° C. to 1000° C. (See e.g. TABLE 6 of Patent Document 2.) The thermistor elements using these sintered materials have a large temperature gradient value (B-value) and become too high in resistance at a low temperature of −40° C. This results in difficulty determining the resistance values of the thermistor elements and enabling temperature measurements.
The thermistor element of Patent Document 3 shows a resistance of 110Ω to 100 kΩ in the temperature range of room temperature to 1000° C. so that the temperature gradient coefficient β (corresponding to B-value) of the thermistor element can be maintained within the suitable range of 2200 to 2480 K. (See e.g. TABLE 1 of Patent Document 2.) No consideration is however given to the relationship between the metal constituent M or M′ of the composite perovskite type oxide (MM′)O3 and the metal constituent A of the metal oxide AOx. Depending on the combination of the metal element M or M′ and the metal element A and the compounding ratio of these constituents, there arises a possibility that the composite perovskite type oxide (MM′)O3 and the metal oxide AOx react with each other to form an unexpected by-product or that the metal element A replaces the constituent element of the composite perovskite type oxide (MM′)O3 to cause a change in composition. This deteriorates various characteristics of the thermistor element (sintered material) such as high-temperature composition stability (heat resistance).